In order to obtain oil from oil shale, the oil shale is directly heated by a hot heat carrier (ash) to a temperature of about 500° C. in a rotary kiln. Hereby, oil evaporates from the oil shale forming the so called vapor gas mixture (VGM). The vapor gas mixture (a gas containing also fine particles) is then quenched in a condensation unit for winning the oil. This oil contains particulate material (fines), which are very hard to separate from the oil and prevent a further improvement of its quality due to e.g. catalyst deactivation. Traditionally, such separation has been done by using a scrubber. The dust particles collected by droplets produced in the scrubber can be found in the cooled oil at the scrubber bottom. If a venturi scrubber is used, there is a high pressure loss, which requires corresponding high pressures in the rotary kiln and thereby increases the equipment costs. Further, dust laden heavy oil is recycled to the pyrolysis zone and thus cannot be used directly as a product. The removal of fine dust particles from oil is a very expensive procedure and a technical challenge which has not yet been completely solved.
According to U.S. Pat. No. 4,548,702 A raw oil shale is fed into a specified surface retort followed by solid heat carrier material at 1000 to 1400° C. The withdrawn product stream is partially dedusted in a cyclone or filter. Further dust is removed in a fractionator, scrubber or quench tower. The oil fraction then is fed into a hydroprocessor followed by a catalyst and hydroprocessing gas. The dust removed from the oil fraction and the water stream of sludge containing the dust is used together with the retorted shale as a fuel to heat the heat carrier material and to retort the raw oil.
From document DE 196 11 119 C2 a process for purifying hot waste gases containing dust and tar and obtained during the production of calcium carbide in an arc furnace is known, which comprises dedusting the waste gas at 200 to 900° C. using a ceramic filter and subsequently removing the tar at 50 to 200° C. using a gas scrubber or electro filter. At such temperatures substantial condensation of heavier oil fractions would have to be expected so that this process is not suitable for dedusting VGM.